High-temperature air oxidation of o-xylene, or naphthalene, over a catalyst such as a molybdenum or vanadium oxide, to produce phthalic anhydride is well-known. Reaction temperatures of 350.degree. to 475.degree. C. with contact times of a half-second to several seconds can be used. Cooling of the reactors is usually carried out by circulating a molten salt. Reactor capacity in this process is, however, typically relatively low because of limitations of the heat removal system in maintaining constant temperature. The reactor effluent is treated to separate the phthalic vapors and the recovered phthalic anhydride is purified. The purification typically is carried out by distillation.
When oxidizing o-xylene to phthalic anhydride a heat amount of 264.8 kcal/mol is released which must be removed in order to control the reaction. Therefore, a tubular reactor is used and is cooled by a fused salt bath, typically consisting of KNO.sub.3 and NaNO.sub.2, which circulates around the reactor tubes. The salt bath is then cooled by heat exchange with evaporating water. One part of the reaction heat is carried away via the circulating fused salt as steam, whereas another part leaves the reactor with the exiting hot reaction gas stream containing phthalic vapors. It is known to utilize the heat transferred to the fused salt bath and the heat carried away by the reaction gas stream for steam production and to use this steam for various heat requirements within the plant and/or to transfer it to a steam supply network for other purposes. From "Hydrocarbon Processing" 53 (February, 1974), pp. 111-112, it is known, for example, to heat water by heat exchange with the hot reaction gas, then to evaporate it in the salt bath cooler, superheat this steam in the reaction gas cooler, and use this superheated steam for the drive of the air compressor. In this method, the compressor is apparently driven by a back pressure turbine, because substantial amounts of back pressure steam at 11 atm. abs. are available. In this operation, a heating or evaporation of a heat carrier suitable for the heating of distillation columns is not provided, and the temperature of the available back pressure steam is not sufficient for the heating of the distillation unit. Using the ordinary organic heat carrier liquids for heating of the distillation unit by heat exchange with the fused salt bath is not desirable for reasons of safety because of the strongly oxidizing effect of this fused salt mixture. A further disadvantage of the known method is that the surplus steam is delivered at a comparatively low pressure.
An object of the present invention is to provide a process for the efficient utilization of the reaction heat generated by the catalytic oxidation of o-xylene with air to phthalic anhydride in a tubular reactor, in which process, in addition to the drive energy for the air compressor and the amounts of heat necessary in the plant, also the heat requirements for the heat carrier liquid needed for heating purposes in the distillation unit of the anhydride recovery stage of the process are provided. Another object of the present invention is to provide a process for the efficient utilization of the reaction heat generated by the catalytic oxidation of o-xylene with air to phthalic anhydride in a tubular reactor in which process surplus steam is to be delivered at a higher pressure and a higher temperature than is possible by the back pressure of a back pressure turbine.